Co, E. L., Rosekind, M. R., Johnson, J. M., Weldon, K. J., Smith, R. M., Gregory, K. G., Miller, D. L., Gander, P. H., Lebacqz, J. V. (1994). Fatigue Countermeasures: Alertness Management in Flight Operations. Southern California Safety Institute Proceedings, Long Beach, 1994, 190-197
By 1980, there was sufficient anecdotal evidence to suggest that fatigue, including jet lag, was a safety issue in aviation. However, there were minimal scientific data to describe the nature or extent of the problem, and therefore almost no foundation on which to base solutions. At that time, Congress requested that NASA determine whether "the circadian rhythm phenomenon, also called jet lag," was indeed a problem in air transport operations (ref. 1). In response to this request, the NASA Ames Fatigue/Jet Lag Program was created. The goals of this Program were threefold: 1) to determine the extent of fatigue, sleep loss, and circadian disruption in flight operations; 2) to determine how these factors affect flight crew performance; and 3) to develop and evaluate countermeasures to mitigate adverse effects and maximize flight crew performance and alertness.
To determine the extent and impact of fatigue in air transport operations, several studies were conducted. Data from field studies conducted during normal operations, full-mission high-fidelity simulations, and controlled laboratory experiments yielded results that integrated the realism of flight deck operations with the accuracy of the laboratory. The findings from these studies indicated that fatigue factors play a significant role in air transport operations (refs. 2-6). In the 14 years since the Program's inception, the methods and measures have been specialized to meet the requirements of studying flight operations and updated to incorporate new technology.
In 1991, the Fatigue/Jet Lag Program became the Fatigue Countermeasures Program, emphasizing the third Program goal. Today, the Program combines expertise in sleep and circadian research with flight operations experience. This unique scope allows the findings from data collected in the field to be returned to the operational community; thus, the industry benefits from the information it provides.
Sleep and Circadian Rhythms in Flight Operations
The term 'fatigue' has been used to describe many different experiences: sleepiness, physical tiredness, inability to focus mentally, and others. The effects of fatigue that concern the operational community-those that affect crewmember alertness and performance-stem primarily from sleep loss, circadian rhythm disruption, and the interaction of the two.
Sleep is a complex process
Sleep, commonly thought of as a time when the brain and body are simply turned off, is actually a complex physiological process. The nature and structure of sleep undergo physiological changes during a given sleep period and alter dramatically over the life span. Further, like many other physiological functions, sleep can be affected by changes in the body and in the environment.
Contrary to popular belief, sleep is not a homogeneous state. Rather, it is comprised of two distinct states: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. These two states differ from each other as much as they each differ from wakefulness. NREM sleep is characterized by slowed physiological and mental activity. Heart rate, breathing, and brain activity slow, and no dreaming occurs. NREM sleep has been classified into four distinct stages, with stage 1 being the shallowest and stage 4 being the deepest sleep. If awakened during stage 1, a person would awaken easily, and quickly adjust to the surroundings; however, waking the person from stage 4 sleep would probably take a few attempts, and the person would most likely be groggy and disoriented for 10 to 15 minutes. This phenomenon is called sleep inertia and is associated with awakening from deep (stage 3 and 4) sleep. During REM sleep, the sleeper is physiologically and mentally active (dreaming), while physically paralyzed. A sleeper awakened from REM sleep would likely be able to describe a dream in detail.
Periods of NREM and REM sleep alternate throughout each sleep period in roughly a 90-minute cycle, with 60 minutes of NREM followed by 30 minutes of REM. Generally, most deep sleep occurs in the first half of the sleep period, while REM periods are longer and more regular later in the sleep period. This structure differs among individuals, and for each individual can be affected by several factors including alcohol intake, prior sleep deprivation, and medication. Understanding the complex architecture of sleep is crucial to determining how to best utilize sleep and nap opportunities on and off duty.
In addition to the changes that occur during a single sleep period, sleep quantity, quality, and structure undergo dramatic changes over the lifetime. With increasing age, the amount of sleep individuals can obtain in a given sleep period decreases, and the quality of that sleep degrades, with sleep becoming less deep and more interrupted. A NASA study found that long-haul flight crewmembers aged 50-60 years lost 3.5 times more sleep per day during trip schedules than their counterparts aged 20-30 (ref. 5). This change in sleep is actually a gradual evolution from birth; for operational purposes, however, the changes that occur over a career are most pertinent.
Sleep is a vital function
No one would give up eating or drinking in order to save time, because they are vital to our everyday functioning and, ultimately, our survival; likewise, sleep is a vital physiological function. Unlike food and water, however, people in our society frequently forego good sleep in order to "make more hours in the day." Although this may seem like a feasible option to a busy person, it is important to acknowledge the toll this behavior can take on alertness, performance, mood, and safety.
Sleep loss, not surprisingly, leads to sleepiness during waking hours. What may be less evident is the fact that sleepiness, in turn, leads to decrements in essentially all aspects of human performance and a worsening of mood. This includes physical, psychomotor, and mental performance: decision-making, response time, judgment, hand-eye coordination, and countless other skills. In everyday life, this can equate not only to scoring poorly on an exam, but also to nodding off while driving a car at 60 mph. In flight operations, these performance decrements can erode even the most conservative safety margin, potentially leading to incidents and accidents.
Further, sleepiness can catch people unaware. One reason for this is that there are two distinct components of sleepiness: subjective and physiological. Subjective sleepiness involves how sleepy someone feels, and can be masked by environmental stimuli, physical activity, and caffeine consumption, among other things. Physiological sleepiness is the body's biological reaction to sleep loss, and can only be reversed with sleep. People's subjective reports of sleepiness often differ greatly from physiological measurements taken. While people tend to underestimate the amount of sleep they get, they tend to overestimate their alertness. Therefore, someone who reports feeling very alert can actually be very sleepy physiologically. Another reason that sleepiness can be deceptive is that sleep loss accumulates into a 'sleep debt.' Therefore, losing even one hour of sleep each night during the week-a seemingly negligible amount-can lead to the equivalent of a 5-hour sleep debt by the weekend.
Even getting the traditionally prescribed 8 hours of sleep does not guarantee peak performance for two main reasons. First, different people need different amounts of sleep, and 8 hours may not be enough; this is due not to laziness or lack of motivation, but to physiology. Second, the benefit of the sleep depends on the quality of sleep, as well as the quantity. Consider a person who normally requires 8 hours of sleep: getting 8 hours of disrupted sleep produces effects similar to getting too little sleep. Many factors can contribute to the disruption of sleep, including: environmental factors (e.g., noise, light, temperature); previous alcohol intake or medication (even non-sleeping pill medications); psychological factors (e.g., family worries, on-duty responsibilities); and sleep disorders.
Alcohol, thought of and used by many as a sleeping aid, actually disturbs normal sleep architecture and disrupts sleep. A NASA study found that short-haul pilots consumed three times more alcohol on trips than at home. They used alcohol (within regulations) to unwind and promote sleep after long duty days that preceded early wake times. However, alcohol does not promote good sleep. It suppresses REM sleep, which leads to REM withdrawal effects and sleep disrup-tion. Further, the effects of alcohol can interact with sleep loss and actually lead to increased sleepiness. Alcohol and the other factors mentioned in the previous paragraph can actually increase the amount of sleep a person requires to be fully alert.
How much sleep does a person need to consistently maximize performance? A person needs the amount of sleep that produces the feeling of being refreshed and alert during waking hours. However, on a night before a big presentation or when there is excessive noise in the room, the same amount of sleep might not provide the usual benefits.
Still another factor determining how sleepy a person feels at a given time is the person's place in the circadian cycle. The circadian cycle creates two times of maximum sleepiness in each 24-hour period, independent of other factors which cause sleepiness. These two windows are roughly from 3-5 AM and from 3-5 PM. More generally, from 12-8 AM, alertness and performance can be decreased.
The 24-hour cycles that bodily functions follow are called circadian rhythms (from the Latin 'circa' meaning 'about,' and 'dies' meaning 'a day'). Since electric light made 24-hour operations and services feasible, there has been a growing demand for the body to digress from its natural internal cycle. While society tends to operate as if people can function equally well at any given time of the day or night, just the opposite is true. In fact, the body generally follows very definite daily rhythms. Under normal circumstances, the brain's clock synchronizes the rhythms of different functions, including sleeping/waking, temperature, excretion, hormone levels, digestion, and most others.
The circadian clock depends on external cues by which to set itself. In fact, in an environment void of these cues, the clock tends to run in approximately 25-hour cycles. However, the time cues, called 'zeitgebers' (German for 'timegivers'), reset the clock daily to maintain the 24-hour cycle. These cues are environmental (mostly sunlight), and social (work/rest schedules and other social interaction).
If the external cues suddenly or drastically change, the circadian clock attempts to adapt to the new cycle, but cannot do so immediately. Jet lag and shiftwork exemplify the problems of requiring the body to change cycles rapidly. Reversing day/night work/rest schedules or flying to a place where night occurs during your body's day poses a problem for the circadian clock. Different rhythms adapt at different rates; therefore, the rhythms will not only be out of synchronization with the new environment, but also with one another. This can lead to symptoms such as disturbed sleep, increased waking sleepiness, degraded performance, and gastrointestinal problems.
Sleep, circadian rhythms, and air transport operations
Sleep and circadian rhythms interact physiologically. Further, air transport operations bring a unique set of circumstances and challenges to these two physiological processes.
Sleep and circadian rhythms interact in several ways. The two factors can work against one another, thereby weakening or negating each other's effect, or they can work in the same direction, thereby intensifying the effect they each have on sleepiness or alertness. When the factors which affect sleepiness (e.g., time of continual wakefulness, prior sleep quantity and quality) favor sleepiness during a low in the circadian cycle (a time of circadian sleepiness), a person trying to maintain wakefulness has both sets of physiological factors to fight. Less dramatic, but important to an industry which requires peak performance during a crisis, are the following possibilities: even a well-rested person who has slept recently can be affected by a circadian low-point; conversely, a person at a peak in the circadian rhythm (which favors wakefulness) can show degraded performance if sleep deprived. Finally, even after a long-duty day, a crewmember may not be able to fall asleep during the rest period if it coincides with a circadian high. Both factors must be considered in determining the likelihood of someone being vulnerable to sleepiness, or oppositely, being at peak performance at a given time.
In addition to interacting with one another, sleep and circadian rhythms interact with the flight operations environment. Flight operations bring distinct factors to bear on the problem: for example, duty at unusual times of day or night, changing schedules, extended duty periods with rest periods at unusual times, and time-zone changes. Like all 24-hour operations, flight opera-tions require people to work at times when their bodies are programmed for sleep. This can lead to increased sleepiness during duty and the associated performance decrements. In addition to requiring unusual sleep/wake times, the duty hours change according to flight schedules and bid lines. This requires a person's body to make changes more frequently. Further, air transport operations often require long duty periods, whether comprised of one extended flight or several short "hops." This increases a crewmember's time of continual wakefulness and, therefore, the chance that underlying physiological sleepiness will surface. Complicating the issue, rest periods often fail to coincide with a crewmember's normal circadian sleep time. This can make the crewmember unable to obtain adequate sleep on layover, and perpetuate the waking sleepiness problem. Finally, many air transport operations entail crossing multiple time zones, which can lead to the circadian disruption previously discussed. All of these factors require the body to adapt to changes in the sleep/wake cycle.
Factors affecting adaptation
Several factors affect an individual's ability to adapt to a new schedule. First, individuals differ in their bodies' ability to adapt to schedule changes; some experience very little trouble, while others take longer and experience more detrimental effects. Second, as mentioned earlier, the ability to adapt decreases with increasing age. Third, the direction of the shift affects the ability to adapt. That is, westward travel (corresponding to a circadian phase delay) is more natural to the circadian clock, and therefore easier, than eastward travel (corresponding to a circadian phase advance). Two cooperative NASA studies showed that crewmembers adapted faster after westward flights than after eastward flights. In one study, crewmembers experienced faster adaptation of sleep and temperature rhythms after flying westward than after eastward flights (ref. 6). A study of long-haul crews flying international flight schedules showed that crewmembers' sleep was less disturbed after westward flights crossing 8-9 time zones than after eastward flights crossing 8 time zones (ref. 3). This study revealed another factor affecting adaptation: crewmem-bers who scored as 'evening-types' also exhibited less daytime sleepiness after eastward flights than those who scored as 'morning-types'. In other words, evening-types ('owls') adapted faster than morning-types ('larks'). Still another factor affecting adaptation is the extent of the time shift, or the number of time zones crossed. The more time zones that are crossed, the longer adaptation takes. Typically, flight crews are not in a time zone long enough to adapt, so they frequently spend the entire layover adapting.
The findings from fatigue studies have suggested many potential countermeasures to the sleep and circadian problems that flight crews face. As with most of the other issues discussed herein, the effectiveness of any countermeasure differs among individuals. Therefore, the countermeasures which follow are suggestions. People will maximize their success by trying different combinations of countermeasures to find what works for them.
A comprehensive approach using two types of fatigue countermeasures can maximize alertness and performance; these are preventive countermeasures and operational countermeasures. Preventive strategies consist of techniques used prior to duty and on layovers to minimize sleep loss and circadian rhythm disruption. Operational countermeasures actively combat fatigue during flight operations.
A crewmember can use preventive strategies to maximize general alertness, so that when flight operations take their toll, the body can respond with its full potential. It has been shown that crewmembers often lose sleep and experience circadian disruption during trips (ref. 2, 5). By getting sufficient, good quality sleep while at home/off duty prior to a trip, understanding how to plan sleep and naps, and developing good sleep habits, a crewmember maximizes the potential for staying alert on duty. Conversely, starting a trip with a sleep debt, little knowledge of how to plan sleep times, or bad sleep habits, makes a crewmember much more vulnerable to increased sleepiness and performance decrements.
The scheduling and quantity of sleep have a strong impact on the ability to sleep, the quality of sleep, and the benefits obtained from the sleep. Crewmembers should get the best possible sleep before trips and start trips fully refreshed and alert to lessen the negative impact of the sleep loss and circadian disruption that may occur during trips. On trips, crewmembers should try to get as least as much sleep during each 24-hour period as they normally do at home. They can schedule off-duty sleep according to personal physiology: feelings of sleepiness indeed indicate the need for sleep, while feelings of alertness or an inability to sleep suggest getting out of bed and resuming normal activity. Scheduled naps can also play an important role in obtaining the sleep necessary to maintain performance.
When large blocks of time are unavailable, naps can be used to augment sleep periods at home or on layover. Studies have shown that naps can acutely improve alertness (ref. 7). When circumstances permit, naps should be taken when a person feels sleepy. The length of the nap depends on the time available. Short naps should be limited to 45 minutes or less, in order to avoid the sleep inertia associated with awakening out of deep sleep. Longer naps should be at least 2 hours to allow for the completion of a full NREM/REM cycle. It seems that no nap is too short; some sleep is generally better than no sleep.
Good sleep habits are beneficial in two ways: one, they foster good sleep at home, which best prepares a pilot for a trip; two, good sleep habits can be used on trips to maximize rest period sleep opportunities. The following recommendations are suggestions based on physiological principles and scientific findings. Develop and follow a pre-sleep routine to promote sleep at bedtime; a warm bath, reading calming material, or just making a ritual of pre-bed preparation can provide the routine. Keep the sleep space and time sacred. Specifically, reserve the bedroom for sleeping and other pleasant activities, and avoid using it for arguments, work, or exercise; also, protect the time set aside for sleep, so that all of that time is actually available for sleep. Make the sleep environment conducive to sleep: maintain dark, quiet, a comfortable temperature, and a comfortable sleep surface. If hungry or thirsty before bed, eat or drink lightly to avoid being kept awake by digestive activity. Avoid alcohol and caffeine prior to sleep. Alcohol can distort sleep architecture, and caffeine can disrupt or preclude sleep. Determine how long before bedtime you need to avoid alcohol and caffeine, and keep in mind that colas, chocolate, and medications (including cold remedies) can contain caffeine. Use physical and/or mental relaxation techniques to promote sleep as necessary. When choosing a technique, remember the following: 1) the technique may require practice before it becomes significantly useful; 2) there are many unsubstantiated claims concerning relaxation techniques, so judge each for yourself. If you can't fall asleep, don't lie awake in bed for longer than 30 minutes; get out of bed and do something that will promote sleep (e.g., read a calming book). On a larger scale, generally healthy diet and exercise habits may also promote good sleep. However, exercise should be avoided too close to bedtime, as it can take time for the body's systems to wind down.
Preventive countermeasures can prepare crewmembers for trips and improve their ability to maximize sleep opportunities on trips. However, preventive strategies alone are not necessarily sufficient to overcome the sleep loss and circadian disruption from long duty periods, time-zone changes, and irregular duty hours.
Operational countermeasures offer crewmembers in-flight strategies for combating fatigue. Presently, in-flight strategies generally mask the effects of sleepiness rather than relieve the physio-logical sleepiness. While only sleep can reverse physiological sleepiness, in-flight strategies can help a sleepy crewmember to maintain wakefulness and a certain level of alertness while on duty.
Operational countermeasures must allow crewmembers to remain in their cockpit seats. FARs require that "each required flight crewmember on flight deck duty must remain at the assigned duty station with seat belt fastened while the aircraft is taking off or landing, and while it is en route" (ref. 8). Sleep deprivation studies have shown that physical activity is the best way to combat fatigue. Therefore, stretching and other physical activity (while limited by the seat restriction) can be used to battle sleepiness. Social interaction can also help mask sleepiness; however, the interaction must be active to have an effect. In other words, while a lively conversation may help the situation, listening and nodding will not. Caffeine can be another useful tool when used properly. Strategic caffeine use calls for avoiding caffeine when already alert (e.g., at the start of duty) and, instead, using it when sleepy or 10-15 minutes before a period of predicted vulnerability (e.g., before the 3-5 AM sleepiness window). As reducing unnecessary physical stresses can help, crewmembers should try to maintain good nutrition and hydration.
While these operational countermeasures can be used to fight sleepiness,
the only way to physiologically reverse sleepiness is to get sleep. Therefore,
a NASA/FAA study examined the effect of allowing flight crewmembers, while
remaining in their seats, to take short planned rests during flights (ref.
9). The naps occurred one crewmember at a time in three-person cockpits
during low-workload (cruise) portions of nonaugmented flights. Some of
the subjects (the Rest Group) were each allowed a sleep opportunity of up
to 40 minutes, while the other subjects (the No Rest Group) were instructed
to carry on with their regular duties during the 40-minute period. While
the No Rest Group showed decrements in alertness and performance at the
end of individual flight legs, after multiple legs, and on night flights,
the Rest Group maintained consistent levels of alertness and performance
throughout. Therefore, the study found that planned and controlled cockpit
naps improve subsequent alertness and performance among crewmembers. While
planned cockpit naps are not currently sanctioned, the FAA is currently
reviewing a proposed Advisory Circular to sanction controlled rest on the
The Single, Ultimate Answer to Fatigue in Aviation
There is none. While a "magic bullet" would be quick, convenient, and less work than understanding principles of sleep and circadian rhythms, such an answer does not exist. To accurately address the complex issues of fatigue in the aviation industry, a comprehensive, eclectic approach is necessary.
Education and Training
Knowing how to select and use fatigue countermeasures requires some understanding of the physiological mechanisms that underlie fatigue. The Fatigue Countermeasures Program, in collaboration with the FAA, has created an Education and Training Module entitled, "Alertness Management in Flight Operations," designed to provide this information in an accessible manner to the operational community. The Program currently disseminates the Module information through workshops held at NASA Ames Research Center. The goal is to provide in-depth coverage of the material to representatives from aviation organizations, so that they can take the knowledge back to their respective organizations and implement programs there. To date, the Module and the workshops have met with much success. Fifty-five representatives from nineteen organizations have attended. Several organizations have implemented the Module; others have developed implementation plans.
Through the Education and Training Module workshops and the subsequent implementation in industry organizations, flight crewmembers and others in the industry (flight attendants, schedulers, managers, and others) will learn about physiological limitations and how to maximize alertness and performance during flight operations. This will increase the industry's overall safety margin. In the future, planned cockpit rest and other new solutions may become available. The Fatigue Countermeasures Program continues to research possibilities for alertness management in flight operations. Those interested in the Fatigue Countermeasures Program or the Education and Training Module workshops may direct inquiries to: Fatigue Countermeasures Program, NASA Ames Research Center, MS 262-4, Moffett Field, CA, 94035-1000.